The negative signaling molecule SH2 domain-containing inositol-polyphosphate 5-phosphatase (SHIP) binds to the tyrosine-phosphorylated beta subunit of the high affinity IgE receptor

The SH2 domain-containing inositol-polyphosphate 5-phosphatase, SHIP, associates with Fc gamma RIIB and negatively regulates both B-cell and mast cell function, We report here that SHIP was tyrosine phosphorylated after high affinity IgE receptor (Fc epsilon RI) aggregation in rat basophilic leukemia RBL-2H3 cells, The ty rosine phosphorylation of SHIP was an early event after receptor aggregation and was present in cells deficient in the protein-tyrosine kinase Syk, Furthermore it was not secondary to the increase of intracellular calcium or the activation of protein kinase C. SHIP was precipitated by immobilized phosphorylated synthetic peptides based on the immunoreceptor tyrosine-based activation motif (ITAM) of the beta but not the gamma subunit of the high affinity IgE receptor. Tyrosine phosphorylation of SHIP and its association with the tyrosine-phosphorylated beta subunit of Fc epsilon RI could play an important role in down-regulating receptor-mediated signal transduction in mast cells, Thus, whereas the activation molecule Syk associates with the gamma subunit ITAM, the beta subunit ITAM binds the negative signaling molecule SHIP. Therefore, unlike B cells where the antigen receptor and coreceptors such as Fc gamma RIIB or CD22 each recruits molecules with opposite effects, the Fc epsilon RI contains subunits which recruit molecules that activate and inhibit signal transduction.